Physics 55Monday, September 12, 2005

1. The celestial sphere, the ecliptic.2. Solar versus sidereal days; synodic versus sidereal months.3. Celestial coordinates: RA and dec4. Understanding the pattern of objects in local skies5. A taste of celestial navigation.

How To Locate Objects in Local Sky

Important words: horizon, meridian, zenith, altitude

Warning: Compasses do not point toward the North Pole, they point to the magnetic north pole which wanders quite a bit over time. Trust the stars, not your compass!

What Appears in Your Local Sky Depends on Your Latitude and Longitude

Will show demo of CD as “local sky” attached to big blue ball as the Earth. Rotating the ball illustrates how the local sky changes with rotation (longitude) and location of the CD on the Earth (latitude).

Here arc FG is the meridian of the local sky that is centered on point P at latitude . Points F and G would be north and south respectively on the horizon of the local sky, while point Z would be the location of the zenith. Line segments PA and PB indicate where the north celestial pole and celestial equator appear in the local sky.

Angular Size, Angular Distance

Fine Angular Measurements:Arcminutes, Arcseconds

60 arcminutes = 1 degree 60’ = 1o

60 arcseconds = 1 arcminute60’’ = 1’

So 3600’’ = 1o

How to pronounce:35o27’15’’

Healthy human eyes can distinguish pointsseparated by about 1’’

What is the Meridian Good For? Definition of “Day”Solar Day Versus Sidereal Day

Solar day: ~ 24 hours on average Sidereal day: 23 hours 56 minutes

Other Trivia

A. M. = ante meridiemP. M. = post meridiem

Note: circadian rhythm is about 25 hours (!) when person lives in deep cave without knowledge of daylight. Why? True for other mammals, animals?

Why Solar Day > Sidereal Day

Earth has to rotate extra degree to align meridian with Sun. Time to rotate 1o is (1/360)x(23 h 56 min) = (1/360)(1436 min) ~ 4 min.

PRS Question: Sidereal Vs Solar Days

If the Earth were to reverse the direction of its orbit around the Sun without changing the way it rotates, then

1. A sidereal day would be longer than a solar day.2. A sidereal day would be shorter than a solar day.3. The two days would have the same duration.4. I have no idea…

PRS Question: What Can Be On Your Meridian at Midnight?

Which of the following statements could be true? I. I saw the planet Venus on my meridian at midnight. II. I saw the planet Mars on my meridian at midnight. III I saw the Moon on my meridian at midnight.

1. I only2. II only3. III only4. II and III.5. None.6. I have no idea.

Other Consequences of Using Sun To Define Time

Synodic month versus sidereal month: familiar month is the synodic month and is defined by when Moon aligns with Sun twice in a row at new moon. Synodic month is 29½ days compared to sidereal month of 27½ days.

More subtle is difference between tropical year and sidereal year but you don’t have to worry about this in this course.

SkyGazer Demo: Motivation for Inventing The Celestial Sphere

Rotation of Stars Around Polaris

Concept of circumpolar stars and circumpolar constellations.

What is circumpolar depends on one’s latitude.

Celestial Sphere

The Celestial Sphere

Ecliptic is path of Sun along celestial sphere: Sun moves easterly w.r.t. the background of stars, taking 1 year to complete its path.

Note how constellations are regions on sphere, not patterns of stars.

Where Does Ecliptic Come From?The Ecliptic Plane and Tilt of Earth’s Axis

Ecliptic of star chart is place to look for planets.

Twelve Zodiacal Constellations

Explains: 1. Origin of zodiacal constellations. 2. Why Sun moves east across the celestial sphere 3. Why planets observed only in, near zodiac Make sure you understand where your meridian points at midnight and at noon.